The use of bags formed of a heat-shrinkable thermoplastic film for packaging primal and sub-primal cuts of meat is well known in the art. In use, to the cut of meat is loaded into the bag. The bag is evacuated to remove air so the bag collapses against the cut of meat and then it is heat sealed to maintain the evacuation. Next the bag is exposed to warm water at about 90.degree. C. or other heating means causing the bag to shrink and form fit the cut of meat. Packaging in this fashion excludes air from the package to prolong shelf life and reduces weight loss due to drying of the meat.
The bag film is relatively thin and usually not more than about 3 to 4 mils (0.076 to 0.10 mm) thick. Accordingly, these bags generally are not suitable for packaging cuts of meat which contain bones. For example, the ribs or other sharp bone protrusions as contained by rib beef cuts or pork loins and other meat cuts may puncture the bag during the evacuation of air or during heat shrinking as the bag draws tightly about the bone-in meat cut. Any puncture in the bag is undesirable as it allows the meat cut in the bag and especially in the area of the puncture to be exposed to the air. The puncture also is a possible source of contamination. The problem of bone punctures is compounded by abrasion during movement of the package along a conveyer and as it is loaded into corrugated boxes and by abrasion between adjacent packages caused by vibration and movement of the meat packages one against another during transport and handling.
One technique for preventing bone puncture is to overlay the protruding bones of the cut of meat with paper, cloth or a wax impregnated cloth prior to insertion into the bag. This is shown for example in U.S. Pat. Nos. 2,891,870 and 4,136,205. Another common solution is to improve the puncture and abrasion resistance of the bag film by adhering a patch to the outer surface of the heat-shrinkable bag. U.S. Pat. No. 4,755,403 discloses use of an oriented heat-shrinkable patch affixed by an adhesive to the surface of a heat-shrinkable bag and U.S. Pat. No. 5,302,402 discloses a non oriented patch adhered to the bag surface by corona treatment. In order to provide the bag with greater protection, U.S. Pat. No. 5,545,419 discloses adhering two heat shrinkable patches to the bag, one to each outer surface of the flattened bag. The patches even may extend over the side edges to insure that coverage extends the full width of the bag.
Neither the cloth nor paper overlay nor a patch adhered to the outer surface of the bag are entirely acceptable solutions to the problem of preventing bone punctures and providing abrasion resistance. One reason for this is that the overlay may be dislocated from its laid-on position as the bone-in cut of meat is inserted into a bag. Patch-bags with one patch require some manipulation of the heavy cut of meat to insure that the patch is properly oriented over the protruding bones. Another drawback of patch bags is the cost of manufacturing the patch material and the added cost of having to laminate one or more patches to the bag. Also, the patch material generally is thicker than the bag. As a result the patch bag tends to be relatively stiff due to its thickness and this stiffness further adds to the difficulty of handling the bag during a packaging operation.
Attempts to avoid applying a patch to the bag have included manufacturing the bag with multiple plies along one side to provide bone puncture resistance. For example U.S. Pat. Nos. 4,704,101 and 5,020,922 disclose heat sealing a wide area of a laid flat tubing to itself to form a double thickness, corona treating one flattened side and then folding the tubing so that the double thickness overlays one of the flatted sides. This forms a triple ply along one side of the bag and a single ply along an opposite side wherein all the adjacent surfaces of the three ply side are interfacially bonded. U.S. Pat. No. 4,481,669 discloses inserting a narrow longitudinally folded web into a wider longitudinally folded web and then heat sealing across the webs to form side sealed bags which have a single thickness adjacent the bag mouth while the rest of the bag has a double thickness. Further, the webs are corona treated so that the plies of the bag adhere one to the other. It is believed that due to the complex nature of the manufacturing process, neither of these bags has a commercial embodiment.
Accordingly, it is an object of the present invention to provide an improved heat shrinkable bag structure and method for packaging bone-in cuts of meat.
Another object of the present invention is to provide a bag for packaging bone-in cuts of meat that has a wall structure which is two plies thick for bone puncture resistance
A further object is to provide a bag structure for packaging bone-in cuts of meat which locates a double thickness of bag film around the entire cut of meat for puncture resistance and yet has only a single thickness in the region of the bag closure.
Still further object is to provide a bag-in-a-bag structure for packaging bone-in meat cuts wherein the inner bag includes at least one opening remote from the bag mouth to facilitate evacuation of air from between the bags.
Yet another object of the present invention is to provide an improved method for packaging bone-in cuts of meat wherein double-bagging is used to provide bone puncture resistance.